The present invention relates to explosives, and in particular explosive emulsions and emulsion/ANFO mixtures. More particularly, the present invention relates to a method of adjusting the rate of detonation for such explosive compositions.
The excavation of rock involves breaking, loading and transportation. Breaking is in most cases accomplished by the use of explosives confined in drill or bore holes or in chambers excavated within the mass of rock to be broken and reached by small tunnels or shafts. It may also be done by undercutting and allowing the weight of the mass to cause caving. Further breaking is caused by the movement of the fallen mass in reaching equilibrium.
The physical characteristics of a rock mass which enter into the breaking problem are the hardness, toughness, brittleness, softness or plasticity of the rock itself and the presence of bedding planes, sheeting planes, joints, cleat or draft in the rock mass. A rock may be both hard and tough or hard and brittle, brittle and soft, soft and plastic, or soft and friable. Soft rocks are easily drilled and broken, while hard tough rocks are difficult to drill and require larger amounts and different kinds of explosives.
The energy of an explosive may be expended in fracturing or shattering a rock mass and in throwing or propelling the broken fragments to a greater or shorter distance. In addition, a certain amount of energy is lost in heating the rock in the immediate vicinity of the charge and in the escape of the gaseous products of the explosion through fissures and seams. The energy expended in breaking and moving the rock mass represents useful work. The property of shattering a rock mass is often referred to as the "disruptive effect", while the property of heaving and throwing is called the "propulsive effect".
The rate of detonation gives a general idea of the disruptive and propulsive effects of a particular explosive. Explosives having a high rate of detonation have a high disruptive effect, while explosives having a very low rate of detonation have a high propulsive effect. For a homogeneous or solid rock mass an explosive of high disruptive effect would be used where the rock is very hard and tough. An explosive of moderate disruptive effect would be used for medium hard and tough rocks, and one of low disruptive effects for soft and brittle rocks. The degree of breaking would be regulated by the amount of powder used and its distribution. For rocks weakened by seams, shear planes and the like, the degree of weakening determines the explosive to be used.
Explosives currently being used in rock blasting situations are generally high shock energy explosives in which all of the explosive energy and the attendant high pressure gases are generated more or less instantaneously. A typical example of such an explosive which is currently used is ANFO, which is a mixture of ammonium nitrate and vegetable and mineral oils with a flash point greater than 140.degree. F., typically diesel oil No. 2. The use of ANFO explosives in many blasting situations results in a number of disadvantages.
As discussed above, an explosive releases energy in two main forms, shock and heave energy. At detonation, there is a sudden increase of pressure that displaces the blast hole wall, generating a strain, or shockwave that produces cracks in the rock. The energy in this wave is of shock energy. After the shockwave is propagated through the rock, the hot pressurized gas which is left in the blast hole is able to extend the cracks as well as to heave the burden. The gas has an energy content referred to as the heave energy. Before blasting, however, rock generally contains sufficient fractures that can be propagated by the heave energy alone. Thus the shock energy serves little or no useful purpose in fractured rock. Furthermore, due to the high shock energy generated by the explosion a greater proportion of fine rock particles are produced by the shock wave. The shock Wave crushes the rock located in close proximity to the bore hole more than is desirable or is required, such as, for use in further processing steps. Minerals or other materials of economic value, such as diamonds, are sometimes damaged by the crushing of diamond bearing rock caused by the shock wave, particularly in locations close to the blast hole.
As a result, the industry has attempted to produce more low shock energy explosives in which more of the energy of the explosive is generated as heave energy and less as shock energy. Such attempts have generally involved dilution of the explosive mixture to produce a lower bulk energy for a given mass of explosive mixture. For example, a mixture of ANFO and sawdust, typically in the ratio of about 2:1, has been utilized. The sawdust acts as a diluent for the ANFO which reduces the density of the explosive mixture. It is well known that the shock energy of an explosive decreases as its density decreases. The problem with reducing the density of the explosive, however, is that in a blast hole the amount of explosive is delimited by the volume of the hole. A low density explosive will not have as much mass in a given volume as a high density explosive. Since the effects of the explosive are related to the amount of explosive in the hole, a low density explosive will not break the rock as effectively as a high density explosive. It would therefore be an advantage to the industry if the heave energy generated could be increased without necessarily lowering the density of the explosive.
In PCT published application WO92/13815, an explosive composition is described which comprises an oxidizing agent such as ammonium nitrate and a fuel material which may include a fuel oil and which comprises a solid fuel such as rubber particles or solid polystyrene beads or flakes. The solid fuel is incorporated into the composition to provide for the controlled release of energy upon detonation of the explosive composition. The published application maintains that by substituting some or all of the liquid fuel oil with a slower burning solid fuel, i.e., the solid rubber particles, the time during which the pressure builds up during detonation is lengthened. Accordingly, a low shock energy explosive is produced having reduced shock energy and increased heave energy compared to conventional explosives such as ANFO.
U.S. Pat. No. 4,820,361 discloses an emulsion explosive containing organic microspheres. The organic microspheres are employed in a water-in-oil explosive emulsion to improve the stability and lower the viscosity. Published Canadian application 2,005,723 also relates to emulsion explosives which includes the sensitizer comprising particles of a compressible material, preferably expanded polystyrene, with said particles having a maximum dimension equal to or less than 3 millimeters, more preferably less than 2 millimeters and most preferably from 0.5% to 7% by volume of the liquid explosive. An alleged advantage afforded by using such sensitizers is that the explosives can be pumped without any significant breakdown of the sensitizing particles, and therefore without unduly affecting the sensitivity of the explosive.
The use of coated thermal plastic microspheres has also been suggested in the prior art. For example, U.S. Pat. No. 4,547,234 describes an explosive composition containing microvoids consisting of thermoplastic resin hollow microspheres coated with a thermosetting resin. The explosive composition is alleged to have a remarkably excellent low temperature detonatability in a small diameter cartridge after lapse of a long period of time. Canadian Patent application 2,042,627 also relates to coating solid additives for water-in-oil and melt-in-fuel emulsion explosives and blasting agents. Described is a coating of a solid which has acid or base sites on its surface with a surfactant having acid or basic characteristics capable of neutralizing the acidic or basic characteristics of the solid surface. Said coating is applied in sufficient quantity to result in neutralization of the acid or base cites on the solid. The result makes the solid additives more compatible with the water-in-oil or melt-in-fuel emulsions and also improves the stability of the water-in-oil emulsion explosives.
The effect of the size of glass microballoons on the detonation velocity of emulsion explosives has been studied, for example, by K. Hattori et al. See, Journal of the Industrial Explosive Society, Japan, 1982, volume 43, No. 5, pages 295-301. In a subsequent paper, K. Hattori et al studied the effects of detonation velocity and ballistic mortar value on underwater explosion performance (shock wave energy and bubble energy) using water-in-oil emulsion explosives whose detonation properties were controlled by the particle sizes or contents of microballoon sensitizers. See, Proceedings of the Thirteenth Symposium on Explosives and Pyrotechniques, Dec. 2-4, 1986. The effective particle size of microballoons on detonation velocity and sensitivity of emulsion explosives was also studied by Hattori et al in a paper given at the Proceedings of the Twelfth Symposium on Explosives and Pyrotechniques, Mar. 13-15, 1984. Glass or silica microballoons of sizes ranging from 33 microns to 566 microns were used in the study. It was concluded that under unconfined conditions, the detonation velocity showed a strong dependency on the microballoon particle size. In contrast, detonation velocity in a confined case, corresponding to infinite explosive diameter, turned out to be practically independent of the particle size.
To date, however, no one has yet achieved an easy and efficient manner of adjusting the rate of detonation of a particular explosive composition without adversely affecting, e.g., decreasing, the density of the explosive composition and/or adversely affecting the sensitivity of the explosive composition. A method and explosive composition which would afford such flexibility would certainly be a great advantage in the technology of blasting.
Accordingly, an object of the present invention is to provide a method for easily and efficiently adjusting the rate of detonation of an explosive composition.
Another object of the present invention is to provide a method for adjusting the rate of detonation of an explosive emulsion composition without detrimentally decreasing the density of the composition.
Still another object of the present invention is to provide an explosive composition which is matched in its rate of detonation with the rock in which it is to be used.
Still another object of the present invention is to provide a method for preparing such a composition which has an adjusted rate of detonation, yet has an effective density and sensitivity as well.
These and other objects of the present invention will become apparent upon a review of the following specification, the drawing, and the claims appended thereto.